This invention relates to a guest-host liquid crystal display device, particularly one of the White & Taylor type.
A guest-host liquid crystal display device which is known in the art employs a pleochroic liquid crystal mixture obtained by dissolving a pleochroic material as a guest in a nematic liquid crystal serving as a host. In such a device, light transmission can be varied by applying an electric field to control the molecular ordering of the liquid crystal mixture. A typical example of such a device has been disclosed in U.S. Pat. No. 3,551,026 issued to G. H. Heilmeier on Dec. 29, 1970.
A disadvantage encountered in this device is a lower level of display brightness owing to the incorporation of an auxiliary polarizer. An improvement upon this device is the display device which utilizes the White & Taylor-type guest-host effect as disclosed in U.S. Pat. No. 3,833,287 issued on Sept. 3, 1974.
In the White & Taylor device a chiral material is added to a pleochroic liquid crystal mixture to impart to the mixture a helical molecular structure in which the helical pitch can be varied freely depending upon the amount of chiral material added. The chiral material may be a cholesteric liquid crystal, a chiral-nematic liquid crystal, or a non-liquid crystal chiral material, etc. While this device affords a high contrast since it allows the auxiliary polarizer to be dispensed with, a serious defect is that the number of turns d/Po must be made greater than 1, where d is the thickness of the liquid crystal layer and Po is the inherent helical pitch. This follows from the fact that the inherent helical pitch Po becomes smaller as the amount of added chiral material is increased to provide the higher contrast. Making d/Po greater than 1 does not permit the display to return immediately to the transparent colored state upon removal of the driving voltage. Instead, there is a "storage" effect in which selected portions of the display exhibit a turbid state which greatly detracts from the effectiveness of the display.